The present disclosure relates generally to radio receivers and transmitters, and more particularly to multichannel radio frequency identification (RFID) transponders and systems. RFID transponders are typically small devices that may include a battery to power internal circuitry, but often operate using energy that can be harvested from an RF field generated by an RFID reader. Low-power transponders often begin operation with zero internal power and generate internal supply power using energy received through a transponder antenna. In operation, the transponder receives a specific RF signal from a reader, and responds by transmitting an RF signal with specific characteristics that can be detected by the reader. The RF signals generated by the reader and transponder are typically modulated with data, allowing data exchange between the reader and the transponder. This feature of low-power RF transponders is useful in a variety of applications, such as automotive devices. RF transponders include an antenna to receive signals and/or energy from a reader, as well as to transmit data to the reader. The data and power reception is typically dependent upon the relative orientation of the reader antenna and the transponder antenna.
Multichannel (e.g., 3-D) transponders are useful where the relative positioning of the reader and the transponder is variable. For example, automotive immobilizer systems include an RFID reader positioned in a fixed location in a vehicle, and a 3-D transponder is mounted to a key fob. A user may be allowed entry to a vehicle and/or permitted to press a start button to start the vehicle if the vehicle's RFID reader properly detects the key fob transponder. However, the relative position of the fixed RFID reader antenna and the key fob-mounted transponder antennas is indeterminate. Accordingly, such immobilizer systems often use 3-D transponder antenna arrangements where a set of three antennas are mounted at mutually orthogonal orientations on the fob. The initial transmission of data from the reader to the transponder is referred to as downlink communications. Once a transponder receives data from the reader, the transponder replies with transmitted information referred to as uplink communications.
The uplink communication is often implemented using frequency shift keying or FSK communications, in which in which the data signal states are represented by two or more discrete frequencies of a carrier signal. The transponder often modulates a capacitor forming part of a resonant circuit that includes the transponder antenna in order to generate multiple RF signal frequencies for FSK uplink transmission. However, such multichannel transponders require multiple FSK modulation circuits, which occupy space on a circuit board or integrated circuit die. Where more than two FSK data states are used for uplink communications, additional modulation capacitors are needed, which further exacerbates the problem.